Effects of heart rate on extracellular [K+] accumulation during myocardial ischemia.

The effect of heart rate on extracellular [K+] ([K+]o) accumulation during total global ischemia was investigated in the isolated arterially perfused (37 degrees C) rabbit interventricular septum using intramyocardial K+-sensitive electrodes (tip diameter, 0.3 mm) and intracellular microelectrodes. The heart rates tested were 0, 25, 75, 125, and 150 beats/min in aqueous-perfused septa and 25, 75, and 125 beats/min in blood-perfused septa. In the lower range of heart rates (0-75 beats/min) the rate and magnitude of [K+]o accumulation during 10 min of ischemia were markedly rate dependent, but in the higher range of rates (75-150 beats/min) the rate dependence was markedly attenuated in both aqueous and blood-perfused preparations. The latter finding could be largely attributed to the more rapid shortening of the action potential duration during ischemia at the higher rates and demonstrates that the amount of time the myocardium spends in the depolarized state is an important determinant of the rate of cellular K+ loss during ischemia. Progressive shortening of the action potential duration during ischemia is also a major factor determining the onset of the plateau phase of [K+]o accumulation. The findings help to resolve previous conflicting reports on the effects of heart rate on [K+]o accumulation during ischemia.